This invention relates to an apparatus and method for performing Laser Shock Peening (LSP). In particular, but not exclusively, the invention relates to the use of a first bubble oscillation period of a cavitation event in determining the effective energy conversion during a Laser Shock Peening (LSP) process.
Conventional Shot Peening (SP) is a cold working process by which compressive residual stresses are introduced into a surface layer of a metal material to improve the mechanical properties. The process of SP typically includes impacting the component surface with particles such as metallic, glass, or ceramic particles to deform the material plastically, thereby changing the mechanical properties. Improvements in laser-based technology offer potential improvements in the SP process which, in turn, offers improvements in the manufacturing sector in terms of enhanced product performance, improved component quality, cost effectiveness and flexible production. Laser Shock Peening (LSP) is a SP process in which compressive residual stresses are induced in the surface layer of metal materials by impacting it with laser pulses instead of the metallic, glass, or ceramic particles used in conventional SP. Mechanical surface treatments such as SP and LSP are commonly used in the manufacturing industry as an effective measure for the enhancement of component fatigue life. There are primarily three factors that affect the operative life of a component, namely fatigue loads, wear, and corrosion. All of these factors can be moderated and controlled by enhancing the mechanical properties of the surface material of a component through the LSP process. It has been found that the fatigue life of a component treated by LSP is several times longer than that of untreated component.
As with conventional SP, performance improvements of the component can be attributed to the introduction of an engineered compressive residual stress through the metallic surface. However, the depth and magnitude of the plastically affected region when using LSP far exceed those of conventional SP. Thus, the emerging technology of LSP has been shown to improve fatigue performance beyond that achievable with conventional SP technology.
The benefits of introducing a layer of compressive residual stress into metallic components generally include increased fatigue performance (lifetime and resistance), resistance to stress corrosion cracking and resistance to fretting related failures. Although conventional SP is a well-established technique, it is limited in its range of applications due to a shallow affected depth of the plastically deformed region and a resultant relatively rough surface finish of the component. LSP technology has developed as an innovative surface enhancement process capable introducing compressive residual stresses to a greater depth and magnitude, as well as achieving a better surface finish than SP. The mechanical impulse generated for peening during the LSP process is due to laser shots from a pulsed laser (as opposed to impacting media as with conventional Shot Peening). When a pulsed laser is fired at a metallic target, mechanical recoil impulse of rapidly expanding vapour and plasma is utilised for permanent material modifications of a component. A schematic illustration of the LSP process is given in FIGS. 1 and 2. An intense pulsed laser beam irradiates a target at power intensities in the range of 1 to 10 GW/cm2. Typically lasers with an output ranging from about 50 mJ to about 50 J are used with a short pulse width in the range of nano-seconds, typically in the region of between 5 and 50 nano-seconds. The incident high intensity irradiation results in vaporisation of the target surface which expands rapidly as a partially ionised gas, which is also referred to as plasma, with a high temperature in the range of 10 000 K and pressure of several Giga Pascals. The rapid pressure pulse due to the plasma expansion generates a shockwave that propagates through the metallic target, resulting in a uniaxial dynamic strain (106 sec−1) and plastic deformation to a depth at which the peak stress no longer exceeds the Hugoniot Elastic Limit (HEL) of the metal (equivalent to the yield strength under shock conditions), which results in a state of residual stress throughout the affected depth.
In an LSP process only a mechanical impact on the work piece is desired. Heating of the material by laser irradiation is kept to a minimum using shorter laser pulses and thermal protective coatings which are also referred to as ablators. The use of laser-absorbent sacrificial coatings has been found to increase shock wave intensity, as well as to protect the surface from laser ablation and melting. By using an ablative coat the surface integrity of the component can be preserved, especially the surface finish. In some applications LSP processes are carried out without using absorbent coatings in what is referred to as Laser Peening without Coat (LPwC). From an industrial perspective, LPwC may be attractive due to elimination of the careful preparation required for application of an absorbent overlay. However the increase in surface roughness may not be feasible for some applications such as the treatment of turbine blades, for example. In addition, the surface degradation may reduce some potential for increased fatigue performance.
Since the primary mechanism of LSP is due to a high pressure pulse generation due to plasma expansion, the LSP process typically employs a confinement regime in order to confine and enhance the magnitude of the pressure pulse delivered to the target by up to 3 orders of magnitude as opposed to freely expanding plasma. Confinement may be achieved by any material sufficiently transparent to laser irradiation, such as quartz for example. However, for practical considerations water is generally used as a confinement medium. A confinement regime wherein water is used as a confinement medium is also sometimes referred to as the Water Confinement Regime or Mode (WCM). The terms “indirect ablation mode” and “confined ablation mode” are also sometimes used to describe a regime in which any confinement medium is used, whereas the term direct ablation is used where no confinement medium is used i.e. when the plasma expands freely in air.
In LSP processes utilising an indirect ablation mode in which water is used as the confinement medium, a nozzle is typically used to deliver a type of water spray or jet to the surface of the component that is being treated. Alternatively, the component that is being treated is submerged completely under water. Schematic illustrations of these two prior art methods are given in FIGS. 3 and 4 respectively.
One of the problems experienced with using a thin water layer or spray as a confinement layer is that air breakdown may occur before the air/water interface. The air breakdown is typically due to the atomisation of water droplets that are ejected out to the atmosphere after each pressure pulse generated after the laser shot. These small water droplets act as breakdown initiation sites due to absorption of high laser intensities. In a commercial LSP process the laser is operated on repetition to fire sequential laser shots at a target for coverage of large treatment areas. The occurrence of air breakdown results in an unknown amount of energy being delivered to the target, thereby reducing process robustness.
The duration until the target area is sufficiently covered with a uniform and laminar water layer thickness is significant as this essentially limits the repetition rate operable during the LSP process. In other words, the fact that the thin layer of water must be given sufficient time to recover before the next laser shot can be fired limits the frequency at which the laser shots can be fired at the target.
Turning now to the prior art method of submerging a component under water, an obvious problem with this method is the size limitations placed on the component by the size of the water bath. Accordingly, in a LSP process employing this method, the range of components that are treatable is limited as some components can simply be treated due to their shape and dimensions.
Since engineered residual stresses can potentially be introduced into any metallic component, there are a multitude of potential industrial applications for LSP. For example, LSP is currently being used in the automotive, marine, power generation, biomedical, and most extensively the aerospace industry. In recently times LSP has also been considered for applications in which tensile residual stresses are a consequence of the manufacturing process, such as subtractive machining methods, including milling, broaching, grinding, laser cutting, as well as welding in joints. However, the commercialisation of LSP is primarily due to the aerospace industry, which remains the market leader of this emerging technology. Typically high value components such as titanium gas turbine blades are treated for enhancements in component fatigue life and resistance to foreign object damage. Recently, there have been developing interests in using LSP technology in integral airframe structural components.
It is an object of this invention to alleviate at least some of the problems experienced with existing LSP processes. It is a further object of this invention to provide a system and method for carrying out an LSP process that will be useful alternatives to existing systems and methods.
In particular, it is an object of the present invention to provide a confinement regime to optimise the shock induced by the laser beam is through the occurrence of a cavitation shock event by maintaining a water layer thick enough for the occurrence of such a cavitation event. It is another object of the invention to measure the first bubble oscillation period of the cavitation event in order to provide a process diagnostic of effective energy transfer to the target.
It is yet another object of the invention to provide a means for creating a water confinement layer with dimensions that are not affected by splashing or water ejection due to the pressure pulse generated. It is yet another object of the invention to provide for the accurate controlling of the thickness of confinement layer so as to optimise the shock effects which introduce the compressive residual stresses. Another object of the invention is to reduce plasma breakdown in the air as well as at the air/water interface before the laser beam reaches the surface that is being treated, thereby resulting in a more repeatable laser energy delivery.